|Relays and Switches|
Switches can be non-locking, i.e. a single button that when pressed makes contact only as long as the switch is pressed, then breaks the circuit when the button is released. These switches are called "push-to-make", see left image. You can also buy switches that work the opposite way, i.e. push-to-break, an example is shown on the right.
Many switches are latching, their state changes permanently depending on the position of the button or lever. Common varieties are toggle switches, slide switches etc. The common on/off switch has a single contact arm and one conatct or position, this is known as single-pole single-single throw or SPST. A switch with a single contact arm and two postions ( 3 contacts ) is called a Single Pole Double Throw SPDT switch, see left image. Often a circuit is required with two sets of contacts. These switches are called Double Pole Double Throw switches (see right image).
In some circuits, e.g. a power supply with multiple outlets, a frequency generator with different output waveforms, a switch with more than two positions is required. A Rotary Switch (see left image) is a prime example of a multi-way switch. Rotary switches can have a single pole (wiper arm) or multiple poles and multiple outlets. The one shown left has 2 poles and 6 outlets, abbreviated to 2P6W.
An expanded view of the contacts of a 2P6W switch is shown below, right.
Not often seen but they do exist,t are multiple pole, multiple way switches. The picture on the left shows a 3 Pole 6 Way switch, the first pole (also called a Gang) is seen nearest the top shaft. If a circuit calls for a 3 Pole 6 Way switch, then one way to substitue this is by using a 1P6W and a 2P6W switch. Using separate switches would require that you turn each switch individually or couple the switch shafts mechanically.
Like switches relays are available in all shapes and sizes. Some relays are miniature and can fit a standard IC socket. The left image is a small DIP miniature relay. The right image shows a DPDT relay that can switch loads of up to 2A at 24 Vdc. Relays that can switch line voltages of 240 VAC and high currents are generally physically larger than the miniature variety.Relays and Contacts
As with switches, relays are always drawn in the OFF or un-operated state. Contacts may be drawn in different ways as shown below; but the middle contact (labeled 2 below) is the wiper arm and will move when the relay is energized. This contact is called the changeover contact, abbreviated CO. The top contact (labeled 1) is the normally closed, NC contact (relay not energized) and contact 3 known as normally open, NO. When power is applied to the relay coil, the NO contact is then mechanically moved and electrically connected to the CO contact.
I will finish this page with a motor reversing circuit.
This circuit uses 4 relays and 3 push buttons for Stop, Forward and Reverse control of a DC motor. RLD has just one changeover contact whereas RLA, RLB and RLC have 2 CO contacts each. Basically RLD will stop the motor, RLA latches the motor in a forward direction and RLB latches the motor in the opposite direction, whilst RLC and its contacts provide the reversing for the motor. This circuit is quite messy and it is common practice to see the relay coil and its associated contacts separated some distance in the actual schematic.The following circuit is identical (electrically) and you should now be able to see that the circuit uses the conventional naming rules of switches and contacts in this article.